1. Field
Exemplary embodiments of the present invention relate to a memory device and, more particularly, to a refresh operation of a memory device.
2. Description of the Related Art
A memory cell of a memory device includes a transistor configured to serve as a switch and a capacitor configured to store charges (or data). A level of data is determined as “high” (logic 1) or “low” (logic 0) depending on whether there is a charge in the capacitor of the memory cell, that is, whether the voltage potential between both terminals of the capacitor is high or low. In theory, there is no consumption of power because the charge should remain in the capacitor. However, data may be lost as the charge stored in the capacitor is lost due to leakage. This leakage is attributable to the PN junction of the MOS transistor. In order to prevent such loss, it is necessary to read data stored in the memory cell, and recharge the memory cell based on the read data, before the data is lost. The only way to retain the data is by repeating the recharging periodically. This process of recharging the memory cell capacitors is called a refresh operation.
The amount of charge leakage increases as temperature increases and decreases as temperature decreases. That is, the data retention time of the memory cells is reduced as temperature increases and increases as temperature decreases. Accordingly, there are techniques for controlling the period for how often refresh operations are executed depending on temperature, as refresh operations need to be performed more frequently when the temperature is high and less frequently when the temperature is low.
As the degree of integration of memory devices is increased, the interval between word lines is reduced. As the interval between word lines is reduced, the coupling effect between adjacent word lines increases.
A word line toggles between an active state and an inactive state whenever data is input to and output from a memory cell. This induces a coupling effect in adjacent word lines. This may affect the charge of memory cells that are connected to adjacent word lines, resulting in a loss of data. This phenomenon is called a row hammering phenomenon. Due to the row hammering phenomenon, data stored in a memory cell may be lost or altered before the memory cell is refreshed.
FIG. 1 is a diagram illustrating a row hammering phenomenon and illustrating part of a cell array included in a memory.
In FIG. 1, ‘WLK’ corresponds to a frequently activated word line, and ‘WLK−1’ and ‘WLK+1’ correspond to word lines disposed adjacent to the frequently activated word line WLK. Furthermore, ‘CELL_K’ is a memory cell connected to the frequently activated word line WLK, ‘CELL_K−1’ is a memory cell connected to an adjacent word line WLK−1, and ‘CELL_K+1’ is a memory cell connected to an adjacent word line WLK+1. The memory cells CELL_K, CELL_K−1, and CELL_K+1 include cell transistors TR_K, TR_K−1, and TR_K+1 and cell capacitors CAP_K, CAP_K−1, and CAP_K+1. For reference, ‘BL’ and ‘BLB’ indicate bit lines.
In FIG. 1, when the frequently activated word line WLK is activated or deactivated, the voltages of the adjacent word line WLK−1 and the adjacent word line WLK+1 rise or drop due to a coupling phenomenon between the frequently activated word line WLK and the adjacent word line WLK−1 and between the frequently activated word line WLK and the adjacent word line WLK+1, thus affecting the amount of charge stored in the memory cells CELL_K−1 and CELL+K+1. When the frequently activated word line WLK toggles between an active state and an inactive state, the amount of charge stored in the cell capacitors CAP_K−1 and CAP_K+1 of the memory cells CELL_K−1 and CELL_K+1 may increase, and data stored in the memory cells may be lost.
Further explaining the coupling effect, electrons are pushed into or drained from the capacitors of memory cells of adjacent word lines or due to electromagnetic waves generated when a word line is toggled between an active state and an inactive state.
The method chiefly used to improve the row hammering phenomenon in which data stored in memory cells CELL_K−1 and CELL_K+1 connected to adjacent word lines WLK+1 and WLK−1 is deteriorated due to the frequently activated word line WLK, is to additionally refresh the adjacent word lines WLK+1 and WLK−1 in addition to a normal refresh operation. Such a refresh operation is called a smart refresh operation. In general, the smart refresh operation is executed whenever a set number of normal refresh operations are executed. For example, whenever four normal refresh operations are executed, the smart refresh operation may be executed once.
The data retention time of a memory cell is influenced by temperature, whereas a phenomenon in which data is drained due to the row hammering phenomenon is not related to temperature. Accordingly, there is a need for a technology in which a period of the normal refresh operation is controlled by temperature and the period of the smart refresh operation is controlled regardless of temperature.